This invention relates to a fastener assembly of the type employing a pre-assembled screw fastener and washer member. More specifically, said invention concerns a frusto-conical washer which converges in a direction away from the fastener drive head rather than toward said head, as is the common accepted practice.
Before discussing the present invention in detail, a brief review of the prior art will be beneficial, since upon recognition of the structural features and deficiencies of the prior art, the present invention will be placed in perspective and more fully appreciated. In this regard, pre-assembled fastener assemblies including a washer captively assembled to a screw threaded member, have long been known and used in the art, and are termed generally "sems". Many users, in particular the automobile industry, require such pre-assembled fasteners in numerous installations or joints that must be effected in the manufacture of their products. Specifically, fastener units employing a frusto-conical washer are useful in maintaining a tight joint as they must be compressed upon assembly and maintain an axial force on the screw member to prevent loosening. Also, the washer may include teeth to establish good electrical contact through the engagement of the washer teeth with the workpiece or structural elements clamped with the fastener.
Generally speaking, in the manufacture of preassembled fasteners of the described type, an unthreaded, headed blank is provided and an apertured, frusto-conical washer member is positioned thereon adjacent the head portion. In these prior art designs, the concave and convex surfaces of the washer extend or converge in a general direction toward the fastener head (See FIG. 1). After this initial assembly, the fastener blank is subjected to a thread forming process, such as cold rolling wherein a screw thread is formed on the body portion thereof. This type of threading procedure produces an upsetting of the material and a reworking thereof to produce a desired thread form, as distinguished from a machining operation wherein material is removed from the blank to provide the thread form. In the cold rolling of the thread, the major dimension or crest diameter of the resulting thread form produced is greater than the original cross-sectional dimension of the unthreaded blank, while the minor thread dimension or root dimeter is less than the original cross-sectional dimension. Accordingly, by sizing the washer aperture so that it will freely pass over the unthreaded blank and subsequently forming a thread as discussed above, the resulting major thread diameter will be greater than the aperture of the washer and thus serve to maintain the washer in assembled relation on the screw shank.
It must be kept in mind that the above discussed thread forming takes place with the washer assembled on the shank, adjacent the head portion of the screw member. Therefore, where a frusto-conical washer is employed, the external thread thus formed can only extend along the shank to a location coincident with the plane defined by the outer peripheral edge or rim of the washer when it is engaged against the drive head. That is to say, the total axial dimension of the frusto-conical washer defines the limit of the extent of engagement of the thread rolling dies along the shank of the screw member. Thus a substantial unthreaded shank portion remains, which is of a diameter between the root and crest diameter of the thread.
The unthreaded shank portion presents a problem in the clamping of relatively thin structural elements. More specifically, the fastener can be engaged with a nut or workpiece only to the extent of the threaded portion thereof, as the mating female threads of said nut or workpiece cannot pass over the unthreaded section. Thus, where thin structural elements are to be clamped, the fastener may bottom before the desired clamping force is attained. While this problem can be overcome by use of flat washers, the advantages of the resulting, flexible frusto-conical or belleville type washer is sacrificed.
As an additional matter, at numerous installations in an automobile for example, the fastener units of the type discussed above are utilized to clamp one or more workpieces, and additionally to provide electrical contact via engagement of the washer teeth with the surface of the workpiece. This is especially important where an electrical ground is achieved by contact with the frame and/or sheet metal of an automobile. It will be appreciated, then, that such grounding necessitates a firm engagement and often a slight biting into the surface of the work piece by the washer teeth, as the workpiece may be covered with a thin film of oxide or the like which would otherwise prevent the desired grounding contact. The residual forces necessary to such engagement are obtained primarily, in the prior art fasteners, only upon complete deflection of the washer to a generally flat position, since it is only when this is achieved that the fastener head will bear against the washer proximate the washer teeth. Therefore, should only partial deflection be obtained, the desired ground contact may not be achieved.
One prior art solution to the above problem concerning the clamping of thin material has been to form the thread prior to assembling the washer with the screw member, thus forming the thread closer to the head of the screw member. However, this requires that the washer member be provided with an aperture large enough to be engaged over the external thread form, whereby the material of the washer must be subsequently deformed or staked to reduce the effective aperture dimension below that of the external thread form, to retain the washer captive on the screw member. Another solution to this problem is as disclosed in U.S. Pat. No. 3,913,649, wherein the shank includes a recluse section to permit thread run out. The solution, however, gives rise to additional problems in that they require additional assembly and thus additional expense.
The present invention solves many of the foregoing problems inherent in the prior art, and does so while retaining both ease of manufacture, relatively low cost and dependability.
Keeping in mind the above discussion, the present invention provides a fastener assembly wherein a frusto-conical washer is pre-assembled with a screw member prior to thread rolling. The washer may include teeth, or some other form of engagement means, or it may be smooth. Further, the frusto-conical form of the washer is reversed with respect to the conventional design, in that it converges away from the drive head rather than toward said drive head. Surprisingly, this design change produces some rather unexpected, synergistic results. Where the washer is of a larger diameter than the drive head, it can be moved closer to the head with the drive head received within the concave portion and the rim extending away from the shank. Thus, thread rolling can extend to a location proximate the drive head. In the embodiment wherein the washer includes teeth, and the washer and drive head are of approximately equal diameter, the overall axial dimension of the washer with respect to the screw body is decreased from the prior art arrangement by the height of the teeth, allowing the shank to be threaded somewhat closer to the head portion. Further, with this design, the head portion bears directly over the teeth of the washer member at all times during assembly, thus encouraging the desired biting engagement of the teeth with the workpiece even though the washer is not compressed totally.
The foregoing, as well as other features and advantages of the present invention will be more fully realized upon consideration of the following detailed description, together with the accompanying drawings, wherein like reference numerals throughout designate like elements and components.